Methods to identify different particles in a field of view are well-known in the art. See, for example, U.S. Pat. No. 4,175,860. Typically, the particles that are under examination are biological particles, and it is desired to automatically identify the different particles under examination.
One of the problems faced by the prior art is that various parameters (such as color) associated with each particle can vary from one sample to another sample. This variation can be caused, for example, by different stains used in the preparation of different samples or, by the age of the samples. Thus, the same type of particle appearing in different images of view may exhibit different parameters.
One solution that has been offered by the prior art is to add a normalizing substance which has a predetermined, a priori detection level. Thus, for example, calibrator particles, as disclosed in U.S. Pat. No. 4,338,024 can be added to a sample. Based upon the a priori knowledge of the color of the beads, if the color of the particle under examination exceeds that threshold, then that particle is identified as of a particular type. Clearly, such a normalizing substance is inadequate because the parameter of the particle can vary from sample to sample (due to the factors of different stain and aging as discussed above), whereas the parameter of the normalizing substance does not.
In another example of the prior art solution, the DNA of all the cells are measured. A normalizing value which is manually selected for the particular sample lot for that experiment is selected and that threshold is then used to identify the particles whose DNA exceeds that threshold or is below the threshold. With this approach, while the threshold can vary, it requires manual interpretation and intervention to determine the particular threshold adequate for each of these experiments.